The on/off-keying (OOK) modulation technique was sufficient for data rates up to about 10 Gb/s per channel in fiber optic communication systems. The capacity of such systems, particularly for long-haul communications, has also been increased by the introduction of wavelength multiplexing, dispersion compensation, error correction, and other advancements. However, to continue to extend the capacity of fiber optic communication systems, the use of new modulation techniques is necessary. The use of a phase shift keying (PSK) modulation technique, for example, rather than OOK modulation, can increase the data rates of fiber optic communication systems but requires more sophisticated detection and equalization techniques.
Non-coherent modulation techniques do not require a reference clock signal at the receiver that is phase synchronized with the carrier signal at the transmitter. Modulation symbols are asynchronously transferred in non-coherent systems, and OOK modulation is one example of a non-coherent modulation technique. On the other hand, coherent modulation techniques, such as PSK, rely upon a phase synchronized reference clock signal at the receiver to increase data rates.
The advantages of using coherent modulation techniques include the detection of quadrature and polarization modulated formats, linear correction, better filtering, and increased sensitivity. Both modulation and demodulation of data in various coherent formats can be performed using optical components combined with electronic digital signal processors, but require more sophisticated detection and equalization techniques. Coherent demodulation also relies upon relatively higher precision and symmetry in the optical devices used for demodulation, and it is particularly difficult to achieve precision and symmetry in smaller, integrated optical devices.